Manufacture of carbon composites by hot pressing

ABSTRACT

A mixture of carbon-containing fibers, such as mesophase or isotropic pitch fibers, a suitable matrix material, such as a milled pitch is compressed while resistively heating the mixture to form a carbonized composite material. Preferably, the carbonized material has a density of at least about 1.30 g/cm 3 . Preferably, the composite material is formed in less than ten minutes. This is a significantly shorter time than for conventional processes, which typically take several days and achieve a lower density material. A treating component may be impregnated into the composite. Consequently, carbon composite materials having final densities of about 1.6-1.8 g/cm 3  or higher are readily achieved with one or two infiltration cycles using a pitch or other carbonaceous material to fill voids in the composite and rebaking.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to a method for forming carboncomposites having a treating component suited for use asfriction-bearing and structural materials for high temperatureapplications. In one embodiment, the composite finds particularapplication in conjunction with a composite material formed byresistance heating of carbon fiber/binder mixtures during application ofa compressive force and will be described with particular referencethereto. It should be appreciated that the method has application inother areas where the combined effects of pressure and temperature aredesired.

2. Discussion of the Art

Carbon composites, such as carbon/carbon composites, include thosestructures formed from a fiber reinforcement, which itself consistsprimarily of carbon, and a carbon matrix derived from a thermosettableresin, such as a phenolic resin or a thermoplastic binder, such aspitch. Such materials are useful in applications where high temperaturefrictional properties and high strength to weight ratio are important.For example, carbon/carbon composites are known to be effective forproviding thermal barriers and for friction bearing applications.Carbon/carbon composites for such applications tend to exhibit goodtemperature stability (often up to about 3000° C., or higher), hightemperature friction properties (typical coefficients of friction are inthe range of 0.4-0.5 above 500-600° C.), high resistance to thermalshock, due in part to their low thermal expansion behavior, andlightness of weight. Thermal insulation materials formed from certaintypes of carbon fibers exhibit excellent resistance to heat flow, evenat high temperatures.

Common methods of forming carbon/carbon composites begins with lay-up ofa woven fiber fabric or pressing a mixture of carbonized fibers derivedfrom pitch (e.g., mesophase pitch or isotropic pitch), cotton,polyacrylonitrile, or rayon fibers, and a fusible binder, such as aphenolic resin or furan resin (the resin process) or needling to holdthe fibers together in a preform (‘dry’ perform process). In the resinprocess, the fibers are first impregnated with resin to form what iscommonly known as a prepreg. Multiple layers of the prepreg or randomfiber prepreg are assembled in a mold of a heated press. The prepreg iscompressed while simultaneously applying heat to the mold attemperatures of 160°C.-180° C. for a period of one hour or more to curethe resin fully. The fiber and cured resin composite is then heated at aslow rate to a final temperature of about 800° C. in a separateoperation to convert the binder to carbon. This carbonization step iscarried out in an inert atmosphere and often takes several days tocomplete. Typically, the density of the carbon composite thus formedranges from about 0.6 to 1.3 g/cm³.

For applications such as brake components and other friction-bearingapplications, a density of about 1.7 g/cm³ or higher is generallydesired. To reduce voids and increase its density, the carbon compositeis infiltrated with a phenolic resin or other carbonizable matrixmaterial using a vacuum followed by pressure and the infiltratedmaterial is then carbonized by heating. Densification is also oftenaccomplished by chemical vapor infiltration (CVI) or chemical vapordeposition (CVD). The chemical infiltration process is generallyrepeated three to five times before the desired density is achieved. Aprocessing step may include graphitization of the preform by heating itin an inert atmosphere to a final temperature not exceeding about 3200°C. Above this temperature, carbon from the composite material tends tovaporize. The graphitization may be a final processing step or anintermediate step.

The lengthy heating and infiltration times render such compositesexpensive and impractical for many applications. For example, it maytake about five months to form a carbon/carbon composite article,depending on the number of densification steps. Accordingly, sinteredmetal articles are commonly used for thermal applications, despite theirgreater weight and often poorer thermal stability and frictionproperties.

The present invention provides a new and improved method of forming adense carbon composite, which overcomes the above-referenced problemsand others.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method offorming a composite material is provided. The method includes combiningcarbon-containing fibers, a carbonizable matrix material, and a frictionadditive to form a mixture and heating the mixture to a sufficienttemperature to melt at least a portion of the matrix material. Theheating step includes applying an electric current to the mixture suchthat heat is generated within the mixture. While heating the mixture, apressure of at least 35 Kg/cm² is applied to the mixture to form acompressed composite material.

Aspects of the invention include a second embodiment of adding afriction additive to the carbon/carbon (“C/C”) composite. This aspect ofthe invention includes combining carbon-containing fibers and acarbonizable matrix material to form a mixture and heating the mixtureto a sufficient temperature to melt at least a portion of the matrixmaterial. The heating step includes applying an electric current to themixture such that heat is generated within the mixture. While heatingthe mixture, a pressure of at least 35 Kg/cm² is applied to the mixtureto form a compressed composite material. The additive is incorporatedinto the compressed composite material by impregnation.

In accordance with another aspect of the present invention, an apparatusfor forming a compressed composite material is provided. The apparatusincludes a vessel, which defines a cavity for receiving a material to betreated. A means for applying pressure applies a pressure of at least 35kg/cm² to the material in the cavity (e.g., a dual action ram or asingle action ram). A source of electrical power generates a current,which resistively heats the material. A temperature detector detects thetemperature of the material. A control system controls the pressureapplying means and the source of electrical current such that themixture is sequentially heated at a first temperature and pressed at afirst pressure for a first period of time, and heated at a secondtemperature higher than the first temperature and pressed at a secondpressure higher than the first pressure for a second period of time.

In accordance with another aspect of the present invention, a method offorming a composite material suitable for vehicle brakes is provided.The method includes compressing a mixture of carbon fibers, a matrixmaterial that includes pitch, and an optional friction additive. Duringthe step of compressing, a current is applied to the mixture. Themixture provides sufficient electrical resistance to the current suchthat the mixture reaches a temperature of at least 500° C. to form acompressed substrate. A carbonizable material is impregnated into voidsin the compressed substrate to form an impregnated preform. The productmay be heated to carbonize the carbonizable material. The impregnationand baking steps are optionally repeated. The impregnated preform may beheat treated to a temperature of at least about 2000° C. to form thecomposite material. Preferably, the composite material has a density ofat least 1.7 g/cc within two impregnation and rebake cycles.

In accordance with another aspect of the present invention, a method offorming a composite material suitable for vehicle brakes is provided.The method includes compressing a mixture of carbon fibers and a matrixmaterial, which includes pitch. During the step of compressing, acurrent is applied to the mixture. The mixture provides sufficientelectrical resistance to the current such that the mixture reaches atemperature of at least 500° C. to form a compressed preform. A frictionadditive is impregnated into the compressed preform. A carbonizablematerial may also be impregnated into voids in the compressed preform.The product may be heated to carbonize the carbonizable material. Thecarbonizable material impregnation and baking steps are optionallyrepeated. The impregnated preform is heat treated to a final temperatureof at least about 2000° C. to form the composite material. Preferably,the composite material has a density of at least 1.7 g/cc within twoimpregnation and rebake cycles.

A further aspect of the invention includes a method of forming acomposite material. The method includes combining a first material, inone embodiment preferably a carbon fibers containing material, acarbonizable matrix material, and an optional friction additive to forma mixture and heating the mixture to a sufficient temperature to melt atleast a portion of the matrix material. The heating step includesapplying an electric current to the mixture such that heat is generatedwithin the mixture. While heating the mixture, a pressure of at least 35Kg/cm² is applied to the mixture to form a compressed compositematerial. The compressed composite material may be impregnated with atreating component.

Another aspect of the invention is a method of increasing the density ofa composite. The method includes the step of combining a reinforcementmaterial which may include carbon-containing fibers with a carbonizablematrix material to form a mixture and heating the mixture to asufficient temperature to melt at least a portion of the matrixmaterial. The step of heating includes applying an electric current tothe mixture to generate heat within the mixture and while heating themixture, applying a pressure of at least 35 kg/cm² to the mixture toform a compressed composite material. The density of the compressedcomposite is increased by introducing a carbonizable material orpyrolytic carbon into voids in the compressed composite and then, ifnecessary, baking the compressed composite to achieve a density of atleast about 1.30 g/cm³. The method further includes impregnating thecompressed composite, having a density of at least about 1.30 g/cm³ witha treating component.

Additional aspects of the invention include a vehicle friction brakeassembly. Preferably, the assembly comprises a friction element havingat least a metal surface. Preferably, the friction element rotates witha wheel of the vehicle. It is also preferred that the assembly includesa braking element having a surface aligned to movably engage the metalsurface of the friction element, wherein at least the surface of thebraking element comprises a carbon composite having a carbonized matriximpregnated with a treating component.

Aspects of the invention also include a method of making a vehiclefriction brake assembly. The method includes a step of rotatablyattaching a friction element comprising a metal surface onto a vehicleand aligning a braking element to movably engage the friction element.Preferably, the braking element comprises a surface comprised of acarbon composite having a carbonized matrix and a treating component,and the surface of the braking element is aligned to engage the metalsurface.

An advantage of at least one embodiment of the present invention is thatcarbon-carbon composites, such as insulation materials or brakecomponent materials, are formed in much shorter periods of time than byconventional hot pressing methods.

Another advantage of at least one embodiment of the present invention isthat the density of the hot pressed material is higher than inconventional preforms, thereby enabling desired densities to be achievedwith fewer densification and carbonization cycles.

Another advantage of at least one embodiment of the present invention isthat a composite material is formed using fewer processing steps.

An additional advantage of the invention is that the carbon/carboncomposite which includes the friction additive has a higher coefficientof friction than the carbon/carbon composite without the additive. Afurther advantage is that the invention may be used to incorporate theadditive substantially uniformly throughout the carbon/carbon composite.

A further advantage of the inventive carbon/carbon composite whichincludes the friction additive is that inventive composite has improvedoxidation stability as compared to a carbon/carbon composite withoutsuch friction additive.

Furthermore, the inventive carbon/carbon composite impregnated with thetreat component has exhibited the advantage of improved machinability ascompared to composites formed by other methods. Also, the composite hasexhibited improved fade/friction properties as well as improved erosionresistance.

Another advantage of the invention is that a carbon composite having acarbonized matrix comprising the treat component may be used as a brakepad on a metal surface of a braking element, such as a rotor or brakedrum.

Still further advantages of the present invention will be readilyapparent to those skilled in the art, upon a reading of the followingdisclosure and a review of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional view of a hot press according to the presentinvention; and

FIG. 2 is a flow chart showing steps of an exemplary process scheme forforming a carbon/carbon composite material having the additive accordingto the present invention.

FIG. 3 is a chart of the fade test characteristics of a composite withthe friction additive, the same composite without the friction additiveand a commercially available product.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The invention will now be described in terms of a carbon/carboncomposite. However, the invention is applicable to any carbon compositewhich comprises a carbonized matrix and second material or materials. Amethod of forming a carbonaceous material suitable for use in thermalapplications, such as friction components, employs resistance heating ofa mixture of an optional carbon reinforcement material, such as carbonfibers, and a matrix material, such as powdered pitch. Optionally, themixture may also include any one of various types of additives (theadditives may also be referred to as a ‘filler” or performance enhancer)or the additive may be added to the carbonaceous material after formingthe mixture. The resistance heating step is accompanied by applicationof mechanical pressure to densify the mixture. After hot-pressing, thecompressed composite or “preform” is preferably subjected to one or moreinfiltration steps employing a carbonizable resin or pitch to increasethe density of the composite material. The densified preform is thenheat-treated to a final temperature of up to about 3200° C. to removeremaining non-carbon components, such as hydrogen and heteroatoms (e.g.,nitrogen, sulfur, and oxygen), and form a carbon/carbon compositematerial, which is almost exclusively carbon. Heat treat may also beperformed between densification cycles.

An exemplary hot press 10 suited to resistively heating and compressingthe mixture is shown in FIG. 1. The hot press includes a mold box 12,which defines a rectangular cavity 14, shaped to receive the mixture 16of fibers, optional additive and matrix material. The cavity issurrounded on four sides 18 by a block or panels 20 of an insulationmaterial, such as a refractory material, which is both electrically andthermally insulative. Pressure is applied to the mixture by upper andlower pistons 22, 24, which are pushed toward each other by applicationof a compressive force to one or both of the pistons. It will beappreciated that the compressive force may alternatively or additionallybe applied from opposed sides 18 of the mixture. Alternatively, pressuremay only be applied by one of pistons 22 or 24. In the case thatpressure is applied by only one piston, the press may be referred to asa single-action ram. The press illustrated in FIG. 1 may be referred toas a dual action ram for at least the reason that pressure is appliedfrom two pistons 22, 24.

A hydraulic system 30, or other suitable system for applying pressure tothe piston(s) 22, 24 urges the pistons together. A resistive heatingsystem 32 applies a current to the mixture. The resistive heating systemincludes first and second electrodes, which are in electrical contactwith the mixture. In a preferred embodiment, the pistons 22, 24 alsoserve as electrically conductive members, i.e., as the first and secondelectrodes, respectively, and are formed from an electrically conductivematerial, such as steel. In an alternative embodiment, the electrodesare separate elements, which may apply the current from the samedirection as the pistons 22, 24, or from a different direction (e.g.,through the sides 18 of the hot press).

The resistive heating system 32 includes a source of electrical powerfor providing a high current at low voltage, such as an AC supply 40.High DC currents are also contemplated. The AC or DC supply iselectrically connected with the electrodes 22, 24 by suitable electricalwiring 42, 44. The mixture of optional additive, matrix material, andfibers 16 is sufficiently conductive to allow current to flow throughthe mixture and complete an electrical circuit with the first electrode22 and second electrode 24 and power source 40, while having sufficientelectrical resistance to generate heat within the mixture 16 as thecurrent flows between the electrodes 22, 24. The heating rate ispreferably at least 100° C./min and can be as high as about 1000°C./min, or higher. The resistance heating rapidly heats the entiremixture 16 to a suitable temperature for removal of volatile materialsand carbonization of the matrix, typically in a matter of a few secondsor minutes, creating voids or bubbles within the mixture. Mechanicalpressure is applied to densify the mixture 16 as the applied heat drivesoff the volatile materials.

The hot press 10 is preferably contained within a chamber 50 of athermally insulative housing 52. An exhaust system (not shown)optionally removes volatile gases from the chamber 50.

The construction of the hot press 10 is such that all parts of themixture 16 within the cavity 14 are subjected to a uniform pressure andto a uniform current flow. This results in the product havingsubstantially uniform characteristics throughout the mass and which issubstantially free of fissures and other irregularities, which tend toresult in fracture during use.

A control system 60 monitors the current applied to the mixture 16 andother parameters of the system. For example, the temperature of themixture 16 is measured with a thermocouple 62, or other temperaturemonitoring device, mounted through the block 20 of the hot press or in apassage in thermal contact therewith. Displacement of the pistons 22, 24relative to each other is detected with a displacement detector 64 fromwhich estimates of the mixture density can be made. The control system60 receives signals from the thermocouple 62 and displacement detector64, corresponding to the temperature and linear displacement,respectively, and measurements of electrical current, voltage across thematerial from the current source 40, and hydraulic pressure from thehydraulic system 30. A processor 66 associated with the controller 60compares the detected measurements with a preprogrammed set of desiredvalues and instructs the control system to adjust certain parameters,such as the applied current, voltage, and/or hydraulic pressure, toachieve a product with the desired characteristics in terms of density,composition, and so forth.

With reference to FIG. 2, a flow chart representing the sequence ofsteps involved in an exemplary embodiment of the manufacture of acarbon/carbon composite material is shown.

In Step 1, a carbon reinforcement material, preferably including carbonfibers, is combined with a carbonizable matrix material and optionally afriction additive. The matrix material acts as a binder and a filler tofill gaps between the fibers. In one certain embodiment, preferably, themixture 16 includes about 20-80% by weight of fibers and about 20-50% ofthe matrix material, more preferably, less than about 40% of the matrixmaterial, and optionally about 0-30% of the friction additive, morepreferably about 3-25% of the friction additive, even more preferably5-20% of the friction additive, by weight. Mixture 16 may optionallyalso include a performance enhancer in about 0-40% by weight.Furthermore, other carbonizable and carbonaceous additives may beincorporated into the mixture. For example, a carbon material, which iselectrically more conductive than the fibers or matrix material, such aspowdered graphitized carbon, may be added to the mixture to increase theconductivity of the mixture if the resistance is too high for current toflow during resistive heating.

Suitable carbon fibers for use as the reinforcement material includethose formed from pitch, such as mesophase pitch or isotropic pitch,polyacrylonitrile (PAN), rayon, cotton, cellulose, other carbonizablematerials, and combinations thereof.

The particular choice of carbon fibers depends on the anticipated enduse of the composite material. For example, mesophase pitch carbonfibers provide the material with good thermal conductivity, oncegraphitized. Composites formed from mesophase pitch carbon fibers thusprovide effective heat sinks for electronic components. Isotropic pitchcarbon fibers exhibit a low thermal conductivity and provide goodthermal insulation. PAN-based carbon fibers exhibit high strength andare thus suited to formation of structural components.

The fibers may be comminuted by a process such as chopping and/ormilling. The carbon fibers preferably have an aspect ratio equal to orgreater than 20:1, more preferably, greater than 100:1, a length of fromabout 2-30 mm, and a diameter of about 5-15 microns. Carbonreinforcements may also take the form of continuous filament yarn,chopped yarn, or tape made from continuous filaments and which arereferred to as unidirectional arrays of fibers. Yarns may be woven indesired shapes by braiding or by multidirectional weaving. The yarn,cloth and/or tape may be wrapped or wound around a mandrel to form avariety of shapes and reinforcement orientations. For ease of handling,bundles of chopped filaments of about 0.2 cm to about 3 cm in length arepreferred. Each bundle may comprise about 200-20,000 fiber filaments,each filament having a diameter of about 5-15 microns. Optionally, thebundles could be of different lengths, with some bundles havingrelatively longer fibers (e.g., 2-3 cm in length), while other bundleshave relatively shorter fibers (e.g., 0.2-1.0 cm in length). As usedherein, the term “fibers” is intended to encompass all elongatedcarbon-containing. reinforcement materials having a length which is atleast twenty times, more preferably, at least 100 times the fiberdiameter.

As for fiber properties, in one embodiment it is preferred that thefiber has at least one or more of the following properties: (1) strengthof at least 500 MPa; (2) modulus of at least 40 GPa,; (3) resistivity ofno more than 50 μΩ-m; and (4) thermal conductivity of at least 8 W/mK.

Exemplary fibers include mesophase pitch carbon fiber, obtained fromMitsubishi Chemical Corp., 520 Madison Ave., New York, or CytecIndustries Inc., 5 Garrett Mountain Plaza, West Patterson, N.J. 07424,and PAN carbon fibers from Zoltek, Companies, Inc., 3101 McKelvey Rd, StLouis, Mo. 63044, or Toray Industries (America), Inc., 600 Third Ave.,New York N.Y. 10016.

The matrix material provides an independent source of carbon uponthermal decomposition. The matrix material is fusible (i.e., capable ofmelting) and contains both volatile and non-volatile components. Thematrix material decomposes on heating to form an infusible materialwhich is primarily carbon with the release of volatiles. Matrixmaterials which may be used to form carbon/carbon composites includeliquids and solids which become sufficiently liquid or have low enoughviscosity upon melting to coat the fibers. Preferred matrix materialsare finely comminuted solids. However the invention is not limited tothe use of finely comminuted solids, non-finely comminuted solids mayalso be used to practice the invention. Exemplary matrix materialsinclude carbonizable thermoplastic resins (such as pitch) furan resins,and phenolic resins. Powdered pitch is a particularly preferred matrixmaterial. Mesophase pitches and isotropic pitches with carbon yields of60% or higher, more preferably, 70% or higher upon coking areparticularly preferred as matrix materials. These pitches are producedfrom petroleum or coal tar, although it is also contemplated that thepitch matrix material may be synthetically formed. Pitch/sulfur mixturesare also suitable as matrix materials. While the matrix material isdescribed with particular reference to milled pitch powder, it will beappreciated that other matrix materials are also contemplated. However,for matrix materials with lower carbon content, such as phenolic resins,it has been found that the quantity of volatile components which arereleased during hot pressing is disadvantageous to forming a product ofhigh density. It has also been found that pitch-based matrix materialsyield a product with improved friction properties as compared with thoseemploying phenolic resins.

The pitch or other matrix material is preferably in the form of a powderor other finely divided material having an average particle size of lessthan about 1000 microns, more preferably, less than 100 microns. Thedesired particle size can be achieved by milling or other comminutionprocess. Exemplary pitch materials include coal tar pitches, availablefrom Rutgers VFT AG, Reilly Industries, Inc., and Koppers Industries,Inc.

The matrix material and reinforcement material may be “dry mixed,” i.e.,mixed without addition of solvents and at a temperature at which thematrix material is still a solid. More preferably, heat is appliedduring the mixing phase to raise the temperature of the matrix materialabove its softening point, which is about 70-350° C. in the case ofpitch (Step 2). Preferably, the mixture is heated to about 30° C. ormore above the Mettler softening point of the matrix material to reducethe viscosity of the matrix material. A Sigrna-type mixer or similar ispreferably used to ensure the fibers and pitch are intimately blended. Ablending time of about 10-30 minutes is generally sufficient.

While the process is preferably carried out in the absence of additionalliquids, such as water or an organic solvent, it is also contemplatedthat a small amount of an organic solvent may be mixed with the matrixand reinforcement materials to act as a plasticizer for the matrixmaterial and reduce the mixing temperature. Other methods, which involveforming a slurry with a volatile liquid and drying the slurry to form apreform, may also be used.

Preferably, the friction additive comprises elements or compounds thatare carbides or react with available carbon atoms to form carbides. Morepreferably the additive comprises at least one of carbides, oxides, andcombinations thereof. Examples of preferred carbides and oxides includecompounds which include at least one of the following elements silicon,boron, titanium, molybdenum, vanadium, chromium, hafnium, zirconium,tungsten, and combinations thereof. Other suitable compounds for afriction additive is an isotropic coke or microcrystalline graphite.

Preferably the friction additive, and optional performance enhancer, isin the form of a particle. Preferably, the particles have an averagesize of at least about 1 micron, more preferably 3 microns or more, upto about several hundred microns, but less than about 1000 microns.Preferably, the size of the friction additive is measured in accordancewith ASTM B 822, titled Standard Test Method for Particle SizeDistribution of Metal Powders and related Compounds by Light Scattering.With respect to the additive, particle as used herein also includes apowder.

The additives may also include performance enhancers. A performanceenhancer is a chemical component of the composite that will improve acertain property of the final article formed from the composite. Forexample, if the final article is a brake component, the performanceenhancer may address properties such as wear or noise. Representativeexamples of performance enhancers include various varieties of bariumsulfate, and disulfides such as, but not limited to, molybdenumdisulfide.

With continued reference to FIG. 2, in Step 3, the mixture of carbonfibers, additive, and pitch powder is optionally packed into a separatemold from the mold box 12 of the hot press and pressed into a brick formhaving a density of about 0.5-1.0 g/cm³ and dimensions only slightlysmaller than those of the mold cavity.

In Step 4, the brick of fibers, additive, and pitch is transferred tothe cavity 14 of the hot press mold box 12 (FIG. 1). In an alternativeembodiment, Step 3, and/or Step 2, is eliminated and the mixture offibers, additive, and matrix material is transferred directly to themold box 12 from the mixer. The lower piston/electrode 24 is raised to aposition in which it forms a base of the mold cavity 14 prior tointroduction of the mixture/brick 16.

In Step 5, pressure is applied to compress the mixture 16. The pressureapplied is partly dependent on the desired final density of thecomposite material. In general, a pressure of at least about 35 kg/cm²is applied. The applied pressure can be up to about 150 kg/cm², orhigher.

In Step 6, the mixture 16 is resistively heated while continuing toapply pressure to the mixture. It is also contemplated that heating maycommence concurrently with, or before the start of application ofpressure. Preferably, both heating and application of pressure arecarried out concurrently, for at least a part of the process time, todensity the material as the volatile materials are given off.

The temperature of the mixture 16 during resistive heating is preferablysufficient to melt the pitch, and optionally remove at least some of thevolatiles from the pitch, and facilitate compression of the fiber matrixmixture as the pitch material is rigidized. It should be appreciatedthat, since pitch is generally not a homogeneous material, a portion ofthe pitch matrix material may remain urmelted (for example, quinolineinsoluble solids tend not to melt), even at temperatures significantlyabove the softening point. Additionally, while substantially all thevolatiles are removed in this step, it is also contemplated that aportion of the volatiles may remain without unduly affecting theproperties of the material.

The mixture preferably reaches a temperature of above the carbonizationtemperature, which is about 500° C. in the case of pitch matrixmaterial. For example, the mixture is heated to at least about 700° C.,more preferably, about 800-900° C., although higher temperatures arealso contemplated. The power input applied during resistive heatingdepends on the resistance of the mix and the desired temperature. For amixture of pitch and carbon fibers, a power input of up to about 60kW/kg is applied, preferably in the range of 45-60 kW/kg, for at leastpart of the heating process. For example, a power input of about 45-60kW/kg is applied for 90 seconds to 2 minutes, which may be preceded byapplication of pressure alone for about 3 to 5 minutes.

In another embodiment, a two-stage process is used. In a first stage(Step 6), a relatively low power input, preferably in the range of about30 kW/kg is applied for a period of about 30 seconds. In this stage, thetemperature is preferably in the range of about 300° C. to 500° C. Thebulk of the volatiles are removed from the mixture in this temperaturerange. Above a certain temperature, about 500° C. in the case of pitchmatrix material, the pitch becomes rigid (carbonizes) and it is moredifficult to remove the volatiles from the mixture without disruption ofthe structure. Accordingly, in the first stage, the temperature ispreferably kept below the curing temperature of the matrix material.

In the second stage (Step 7), the temperature is increased to a highertemperature (e.g., above about 700° C., more preferably, 800-900° C.),sufficient to carbonize the matrix material. In this stage, the powerinput may be from about 45 kW/kg to about 60 kW/kg to bring thetemperature up to about 800-900° C. The power is maintained at thislevel for about 1-2 minutes, or longer. The optimum time depends on theapplied power input, resistance, and other factors

The first and second stages are preferably also associated withdifferent applied pressures. In the first stage (Step 6), for example,the pressure is lower than in the second stage (Step 7). The lowerpressure reduces the opportunity for volatile gases to be trapped in themixture, causing violent disruption of the mixture as they escape. Forexample, a pressure of about 35-70 kg/cm² is employed for the firststage, while an increased pressure of about 100-150 kg/cm² is employedfor the second stage.

The resistance heating/pressing step (Step 6 and/or Step 7) takes underthree hours, preferably, about 30 minutes or less, more preferably, lessthan about ten minutes, most preferably about 5-8 minutes, which is amuch shorter time than the days required in conventionalheating/pressing systems. Additionally, the density of the preformformed in this step is preferably at least 1.3 g/cm³, more preferably,at least 1.4 g/cm³, most preferably, about 1.5 to 1.7 g/cm³. This ismuch higher than the density generally achieved in conventional methods,where the density of the fiber/matrix preform is about 0.6-1.3 g/cm³without further densification procedures. As a consequence, fewerinfiltration cycles (Step 9) are used to achieve a final desired density(generally 1.7-1.9 g/cm³, more preferably 1.75-1.85 g/cm³) with theresistive heating method than with conventional hot pressing methods.This decreases the number of processing steps and reduces the overallprocessing time even further. For example, where six or moreinfiltration steps are commonly used in a conventional process, thepresent process accomplishes a final density of about 1.75-1.85 g/cm³ inonly one or two infiltration steps. Whereas the conventional method maytake several months from start to finished product, the presentresistive heating method reduces the time to a matter of days or weeks.

In step 8, the hot-pressed preform is discharged from the mold cavity 14and cooled. Preferably, the preform is cooled rapidly to a temperaturebelow which oxidation does not occur at a significant rate. For example,the preform is immersed in water or sprayed with droplets or a mist ofwater to bring its temperature below about 400-500° C. Alternatively,cooling may be achieved with an inert gas flow. Depending on theparticular application of the carbon/carbon composite, it may bepreferred that the preform is cooled at a rate to avoid cracking of thepreform.

While the preform is readily formed in the shape of a rectangular brick,it is also contemplated that the mold cavity may be configured toproduce a preform of a cylindrical or other shape, thereby reducing oreliminating the need for subsequent machining to form a desiredcomponent part.

Further densification of the cooled preform takes place in Step 9. Inthis step, a carbonizable material is impregnated into the preform bodyby pitch or resin impregnation. After each impregnation step, the bodyis preferably rebaked in Step 9 to carbonize the carbonizable material.It has been found that a target density of about 1.6-1.8 g/cm³ isreadily achieved with only a single impregnation step. A density of 1.7g/cm³, or more, is readily achieved within two such impregnation steps.In this process, the preform is placed in a vacuum chamber and thechamber evacuated. Molten pitch is introduced to the chamber andpenetrates into the evacuated pores in the preform, with the aid ofapplied pressure.

In step 9, the body is heated slowly in a furnace, for example, at aheating rate of about 10° C./hour to about 20° C./hour to a finaltemperature of about 800-900° C. The body is preferably held at thistemperature for about 2-3 hours and then the power is removed. The bodyis then cooled to a temperature of about 100° C. or less before beingremoved from the furnace. The body may be cooled slowly, such as over aperiod of two to three days depending on the size of the body.Alternatively, a cooling medium such as water may be used to reduce thecooling time by spraying the medium on the body. Each carbonization stepthus takes about 5-6 days to complete. Having fewer infiltration andcarbonization cycles therefore reduces the overall densification time.

In an alternative densification process, the preform is exposed to anatmosphere of a gaseous hydrocarbon, such methane, ethane, propane,benzene, and the like, or a mixture thereof. The hydrocarbon gasdecomposes, or is cracked, for example at a temperature of about 900° C.to about 1,200° C. to form elemental carbon, which is deposited withinthe carbon/carbon composite as a pyrolytic material. This may bereferred to as chemical vapor infiltration (“CVI”).

In the case that the friction additive comprises an oxide, an embodimentof the invention may include a heat treating step, preferably after Step9. Preferably the heat treating comprises heating the compressedcomposite material to a sufficient temperature for a sufficient periodof time to convert at least a portion of the oxide friction additive tocarbide. For example, if the oxide comprises SiO₂, the heat treatingstep comprises heating the composite material to a temperature of atleast about 1500° C., such as at least about 1700° C. to about 1800° C.for a period of up to about 5 hours, such as about 2 to about 4 hours.Preferably, the heat treatment converts at least of portion of the SiO₂into SiC. However, not all of the oxide is required to be converted intoa carbide to practice the invention.

At Step 10, the body is subjected to a heat treatment process. In thisstep, the body is heated in an inert atmosphere, for example, in aninduction furnace, to a temperature of about 1500° C., or higher, morepreferably, about 2000° C., most preferably, about 2400° C., to removeall (or substantially all) hydrogen and other heteroatoms and produce acarbon/carbon composite. In heat treating the body, it is preferred thatthe body is not subjected to a temperature equal to or greater than thedecomposition temperature of the friction additive, e.g., about 2600° C.for SiC. Above about 2400° C., the composite is at least partiallygraphitized. The heat treat temperature is selected according to the enduse of the final product and is generally above the highest temperatureto which the composite material is to be subjected in use.

During this heat treat process, various physical properties of thecomposite material, such as its thermal and electrical conductivity, aresubstantially increased, making the composite material suitable forvarious high temperature commercial applications. The period of time forthis procedure is calculated using conventional calculations based uponheat treat time/temperature kinetics, taking into account furnacethermal load and mass.

The invention further includes an alternative embodiment ofincorporating the additive into the carbon/carbon material. As shown inFIG. 2, the invention may include optional step 8A to incorporate theadditive into the carbon/carbon composite. The friction additive, aswell as the optional performance enhancer, may be impregnated into thecompressed composite material. In the case of adding the frictionadditive by impregnation, suitable forms of the additive include atleast colloidal suspensions and solutions.

Preferably the colloidal suspension comprises the additive in aconcentration of at least about 20% up to about 75%, more preferably atleast about 25% up to about 60% and even more preferably at least about30% up to about 50%. Preferably the additive is in the form of about amicron or smaller particle, more preferably, a submicron sized particle.The additive may be suspended in any material in which the additive isnot soluble and the material can be readily vaporized, e.g., in the casethat the additive comprises SiO₂, water comprises a suitable material tosuspend the additive. The material may be referred to as a liquidcarrier. An example of a preferred colloidal solution of the frictionadditive includes silicon dioxide 30% dispersion in water available fromAlfa-Aesar Co. of Ward Hill, Mass.

In an embodiment, the friction additive is impregnated into thecompressed composite material under vacuum. For example, the compositematerial may be placed in a vessel fitted with a vacuum outlet and thepressure inside the vessel is reduced below about 50 mm of mercury,preferably below about 10 mm of mercury. The friction additive containedin a separate vessel is then introduced through a connecting valve andthe pressure inside the vessel including the composite is increased toatmospheric pressure or higher. Preferably, the composite materialremains completely immersed in the liquid carrier of the frictionmaterial or the solution containing the friction additive for at leastabout 10 minutes at about atmospheric pressure or higher. Theimpregnated composite may then be removed from the vessel for furtherprocessing or Step 8A may optionally be repeated 1 or more timesdepending on the amount of friction additive desired in the composite.At the end of the friction additive impregnation step, any excess liquidin the vessel may be drained off. Also in the case that a vacuum pump isused to create the negative pressure (vacuum) in the vessel, it may bepreferred that the vacuum pump is isolated from the vessel prior tointroducing the friction additive.

Optionally, this embodiment of the invention may include the step ofsubstantially removing the material which the additive is suspended infrom the composite material. For example, if the material is water, thecompressed composite, after impregnation, is dried to remove the water.After the material is substantially removed, the compressed compositecontaining the additive may be processed in the same manner as describedabove.

With respect to the timing of the impregnation of the composite bodywith the friction additive, the impregnation may take place before orafter heat treating and it may also take place before or after theaforementioned carbon densification impregnation of the preform.

Though Step 8A has been introduced above as an alternative to includingthe friction additive in Step 1, if so desired, Step 1 with the frictionadditive may be practiced along with optional Step 8A.

The invention may also include increasing the density of thecarbon/carbon composite by impregnation with a treating component.Preferably treating components include a thermosettable resin, a metal,a metal-alloy, and combinations thereof. Examples of preferred resinsinclude such as but not limited to, phenolic resins, epoxies, urethanes,polyimides, cyanate esters, and furan derived resins. One preferred typeof phenolic resin comprises a “resole” which comprises an alkalinecatalyzed thermosettable phenol-formaldehyde-type resin includingpartially condensed phenol alcohols. Preferably, the formation of theresole takes place in the presence of the alkaline catalyst with aformaldehyde to phenol ratio of greater than about 1, where the methylolphenols can condense either through methylene linkages or throughmethylene ether linkages. A preferred type of epoxy comprises an epoxynovolac. Preferably, the novolac resin is formed in the presence of anacid catalyst with a formaldehyde to phenol ratio of less than about 1.Examples of preferred metals include, at least, aluminum, copper, boronand alloys thereof. The metal may be in the form of a metal containingcompound such as, but not limited to, a metal-halide.

In one preferred embodiment of the treating component impregnation, theimpregnation may take place under vacuum. Preferably, the impregnationof the thermosettable resin may take place at a temperature of aboutroom temperature or higher. The temperature of a metal impregnation ispreferably above a temperature required for the metal to be in a liquidphase. Optionally, the impregnation step includes subjecting thetreating component impregnated composite material to a temperaturegreater than the highest expected use temperature of the compositematerial.

In the case of the use of the thermosettable resin treating component,preferably, the impregnated resin is cured after the impregnation step.Preferably, the resin impregnated composite is heated to a temperatureof about 400° C. or less, more preferably about 300° C. or less to curethe resin. Suitable curing temperature comprise about 250° C. or lessand even as low as about 150° C. Once the resin is cured, the resin maybe referred to herein as a thermosett material.

After curing, the resin provides a non-abrasive character to thecomposite. This is particularly important if the composite is to be usedin friction applications such as for brake pads and rotors or brake padsand brake drums. The resin impregnation of the composite reduces theabrasiveness of the composite and improves the erosion resistance of thecomposite when applied to a metal surface of a rotor or a brake drum.This is a desirable effect brought about by the resin impregnation.

With respect to the timing of the treating component impregnation,preferably, the impregnation step will take place after the carboncomposite material has reached a density of at least about 1.30 g/cm³.Examples of preferred densities for the treat component impregnating thecomposite comprise, at least about 1.45 g/cm³, at least about 1.55g/cm³, and at least about 1.60 g/cm³. The treating componentimpregnation may take place as a final processing step prior tomachining the composite, for example after heat treatment (Step 10 ofFIG. 2). However, the invention is not limited to practicing thetreating impregnation as a final processing step.

Advantages of impregnating the composite with the treating componentinclude an increase in the density and strength of the composites and areduction in porosity. In addition to the advantages of increasingstrength and reduced porosity, the invention may also be practiced toprotect the composite from excess wear and reduce the abrasion infriction applications of the composite. A further advantage of thetreating component impregnated carbon/carbon composite is that it may beused as a braking material, e.g., brake pad, to contact a metal surfaceof a brake rotor or brake drum of a vehicle. Examples of a suitablemetal surface include cast iron, aluminum, or stainless steel.Preferably, the brake rotor or brake drum is part of the hub assembly ofa wheel of a vehicle.

Once the general shape of the carbon composite article is fabricated,the piece can be readily machined to precise tolerances, on the order ofabout 0.1 mm or less. Further, because of the strength and machinabilityof carbon composites, in addition to the shaping possible in the initialfabrication process, carbon composites can be formed into a variety ofshapes.

The resulting carbon composite material is suited to a wide range ofapplications, including use as brake components, antiskid components,and structural components, such as body panels, pistons, cylinders, forvehicles, such as aircraft, high performance cars, trains, and aerospacevehicles, missile components, and for use as susceptors in furnaces. Thereduction in processing time achieved with the resistance heating methodopens up many other applications for the material which have hithertobeen impractical because of time and production cost constraints.

Typical properties of a carbon/carbon composite formed from mesophasepitch carbon fibers and milled pitch are as follows:

As-pressed density of the preform: 1.55-1.65 g/cm³;

Final density after graphitization: 1.75-1.82 g/cm³ (with two pitchimpregnation/carbonization cycles)

Flexural strength: about 50 MPa

Young's modulus: about 35 GPa

Compressive strength: about 60 MPa

Thermal conductivity: about 75 W/m·K.

The electrical conductivity of the graphitized material is generally inthe range of about 9-10 μΩ-m. With the exception of thermalconductivity, these properties were measured perpendicular to the fiberorientation (perpendicular to the pressing direction). Thermalconductivity was measured in the fiber orientation direction.

Without intending to limit the scope of the invention, the followingexamples demonstrate the improvements in processing times and otheradvantages that can be achieved by practicing the invention.

EXAMPLES Example 1 Carbon/Carbon Composite Made by Dry Mixing ofPrecursor Materials

Mesophase pitch-based carbon fibers and a matrix material of milledpitch with 170° C. Mettler softening point (“SP”) (ASTM D 3104) and 70%coking yield were dry mixed at ambient temperature in a Sigma-typeblender or similar type of mixer for about 5-15 minutes. The ratio offibers to pitch matrix material was from 50-80 wt % fiber: 20-50 wt %pitch. The mixture was collected and charged into a mold box cavity(dimensions approximately 23×20 cm) of a hot press, as illustrated inFIG. 1. A pressure of up to about 140 kg/cm² was applied to the mixturein the press. After pressing to compact the mixture, an electric currentof about 1000-2000 amps (a power input of about 30-60 kW/kg) was passedthrough the mixture. The mix was held under the temperature and pressureconditions for about 5-10 minutes. The temperature of the mixturereached 800-900° C. This hot pressing process carbonizes and densifiesthe fiber/matrix mixture in a very short period of time, compared withconventional processes. The as-pressed material (preform) had acarbonized density of about 1.6 g/cm³. The preform underwent two pitchimpregnation cycles, each one followed by re-carbonization, to densifythe material. Lastly, the preform underwent graphitization to atemperature of about 3200° C. to obtain a product having a density ofabout 1.75 g/cm³.

Example 2 Carbon/Carbon Composite Made by Hot Mixing of PrecursorMaterials

Various batches of mesophase pitch-based carbon fibers and a matrixmaterial of milled pitch from Example 1 were hot mixed at a temperatureof about 200° C. in a Sigma-type blender or similar type of mixer forabout 30-45 minutes. The ratio of fibers to pitch matrix material wasvaried from about 50-80 wt % fiber: 20-50 wt % pitch. During the hotmixing, the matrix material coated the fibers uniformly. The mixture wascollected and charged into a mold box cavity of a hot press, and heatedand pressed as described for Example 1. Alternatively, the mixture wascompacted in a separate mold to a density of between about 0.5 and 1.0g/cm³ prior to hot pressing.

A pressure of up to 140 kg/cm² was applied to the mixture in the hotpress. After pressing to compact the mixture, an electric current ashigh as 1500-2000 A (a power input of about 45-60 kW/kg) was passedthrough the mixture. The mix was held under the temperature and pressureconditions for about 5-10 minutes. The temperature of the mixturereached 800-900° C. This hot pressing process carbonizes and densifiesthe fiber/matrix mixture in a very short period of time, compared withconventional processes. The as-pressed material (preform) had acarbonized density of between about 1.4 and 1.65 g/cm³. The preformunderwent two pitch impregnation cycles, each one followed byre-carbonization, to densify the material. Lastly, the preform underwentgraphitization to a temperature of up to about 3200° C. to obtain aproduct having a density of about 1.70 to 1.75 g/cm³.

TABLE 1 shows the as-pressed densities obtained for various fiber andpitch compositions (i.e., prior to infiltration and graphitization).TABLE 1 Carbon Fiber (wt %) Pitch Binder (wt %) As-pressed density(g/cc) 75 25 1.61 65 35 1.56 55 45 1.37 45 55 ** The block cracked after hot pressing.

As can be seen from TABLE 1, the as-pressed density decreased as thepitch binder concentration was increased. Thus for applications wherehigh as-pressed density is desired, it is preferable to keep the pitchbinder concentration below about 40-45%.

Samples of the as-pressed composites having an as-pressed density ofabout 1.55-1.64 g/cm³ were subjected to two impregnation/carbonizationcycles. In each impregnation step, petroleum pitch was impregnated intothe composite. The samples to be infiltrated were first heated to atemperature of about 250° C. for 6-8 hours and then placed in a pressurevessel, which had been preheated to at least 200° C. A vacuum was pulledfor 4-6 hours and then the vacuum pump was isolated and liquid pitch wasintroduced to the pressure vessel. Nitrogen was introduced into thevessel at a pressure of 100 psig and the samples were impregnated withthe liquid pitch for 10-12 hours. The pressure was then released fromthe vessel and the impregnated composite samples retrieved after theexcess liquid pitch was removed.

The impregnated composite was then carbonized by heating it in a furnaceto a temperature of about 800-900° C., using a heating rate of 10°C./hour. The temperature was held for about 2-3 hours. The power wasremoved and the composite allowed to cool from about 900° C. to about100° C. over a period of two to three days. The impregnation andcarbonization steps were then repeated. The carbonized composite wasthen graphitized in an induction furnace by heating the material to atemperature of 3000° C. at a heating rate of 300° C./hour. The finaltemperature of 3000° C. was maintained for approximately one hour. Testson the graphitized composite material produced the following results:

Final density after graphitization: 1.75-1.82 g/cm³

Flexural strength: about 50 MPa

Young's modulus: about 35 GPa

Compressive strength: about 60 MPa

Thermal conductivity: about 75 W/m·K.

Electrical conductivity: about 9-10 μΩ-m.

Example 3 Silicon Carbide Carbon/Carbon Composite Made by Hot Mixing ofPrecursor Materials

Direct Addition of Solid SiC.

Blends of silicon carbide powder with an average particle size of about10-20 microns from Alfa Aesar Company were blended with mixtures ofchopped mesophase pitch carbon fiber and a 170° C. SP coal tar pitch bystirring at about 220° C. The mesophase pitch carbon fibers were about¼-inch long obtained from Mitsubishi Chemical Corp. The following blendcompositions were prepared, as shown in TABLE 3-1: TABLE 3-1 1 2 3 4Carbon fiber 77% 69%   69% 62.5% Matrix 19.2%   27.6%   17.2%   25% SiC3.8%  3.4%  13.8% 12.5%

The blends were hot pressed to produce carbonized 3 cm thick carbonizedcomposites with dimensions of 23 cm×23 cm. The densities measured forthe as-pressed composites were: (1)=1.52, (2)=1.53, (3)=1.55, and(4)=1.55 g/cc. The composites impregnated with petroleum pitch and thenrebaked to 900° C. and the impregnation and rebake steps were repeated.After the 2impregnation/rebake cycles, the composites were graphitizedby heating to about 2500° C. Final densities for the composites were:1.72 g/cc for (1) and (2) and 1.75 g/cc for (3) and (4). The finalcomposites contained about 4 and 16% of silicon carbide respectivelydistributed uniformly throughout the composite.

Additional composites were produced as in Example 3 using a blend of 10parts of SiC powder to 25 parts of the 170° C. SP pitch to 65 parts ofchopped mesophase pitch carbon fibers. One composite artifact wassubjected to 1 petroleum pitch impregnation (“1 PI”) followed by rebakeand graphitization while a second composite was subjected to 2 pitchimpregnation/rebake cycles (“2PI”) followed by graphitization. The finaldensity for the 1 PI composite was 1.67 g/cc and 1.73 g/cc for the 2 PImaterial.

Friction Testing of the SiC/C/C Composite

Both composites were subjected to friction testing using the Chase Test.The Chase Dry Friction Machine is described in the web site of GreeningTesting Laboratories Company of Detroit, Mich. where the test wascarried out. As shown in FIG. 3, the test results for the SiliconCarbide Carbon/Carbon Composites (“SiC/C/C”) of 2 PI is compared to aCarbon/Carbon (“C/C”) composite produced by the same process but withoutthe addition of silicon carbide (see example 6). Without the SiC thecoefficient of friction is low at low temperatures, ˜0.1 at 100° C., orless, and remains lower than the composite with the friction additivethroughout the test, although the friction performance of the compositewithout the friction additive does improve with increasing temperature.For the SiC/C/C composites, the coefficient of friction remains high atabout 0.3 at low temperatures and rises to above 0.4 at hightemperatures. The friction behavior is close to that shown for acommercial carbon/metallic brake material used in racing cars.

Example 4 Silicon Carbide Carbon/Carbon Composite Made by Hot Mixing ofPrecursor Materials

Addition of Silicon Dioxide Followed by Thermal Conversion to SiC

A hot-pressed composite was produced by adding about 10 parts of SiO₂powder (˜2 microns from Alfa Aesar Co.) to 25 parts of the pitch binder175 SP and 65 parts of fibers. The composite was then impregnated oncewith petroleum pitch and rebaked to ˜900° C. The density after rebakewas 1.65 g/cc. The impregnated composite was then heat treated up to˜1750° C. and held at that temperature for about 5 hours to effectconversion of the SiO₂ to SiC by reaction with the carbon in thecomposite. The composite was then taken to ˜2500° C. and held at thattemperature to effect graphitization. The final density of the compositewas 1.46 g/cc. The reduced density is due to the loss of carbon fromconversion of the SiO₂ to SiC. Microscopy examination of the compositeshowed that essentially all the SiO₂ had converted to SiC leaving ˜7%SiC in the composite.

Example 5 Silicon Carbide Carbon/Carbon Composite Made by Impregnation

Impregnation with Colloidal SiO₂ Followed by Thermal Conversion to SiC

Carbon/carbon composites were produced using 33% coal tar pitch binderand 67% mesophase pitch carbon fibers were impregnated with a colloidalsuspension of SiO₂ particles in water. The as-pressed composites whichcontained no other additives had a density of ˜1.54 g/cc after the 900°C. heat treatment in the pressing step. The SiO₂ was obtained from AlfaAesar Co. as a 30% aqueous suspension of 0.01 micron particles. Theimpregnations were carried out at 25° C. by subjecting the composites toa vacuum of about 0.4 mm. The pump was isolated and isolation wasfollowed by introduction of the SiO₂ dispersion in water and thenequalization of the pressure to 760 mm. The composites were then driedin a vacuum oven by heating slowly to remove the water. The finalpressure in the oven was about 0.1 mm and the final temperature wasabout 175° C. The weight pickup of SiO₂ was measured as ˜4.5%.

One of the composite samples was subjected to a second impregnation withthe SiO₂ dispersion and after water removal the total SiO₂ pickup wasmeasured as ˜7.5%. Both composite materials were then heated to 1750° C.and held there for 5 hours to effect conversion to SiC. The compositeswere then heated to 2500° C. for 1 hour to effect graphitization. Thefinal composite densities were measure as 1.50 g/cc for the singlyimpregnated composite and about 1.47 g/cc for the twice impregnatedcomposite. Examination by microscopy confirmed the conversion of theSiO₂ to SiC had occurred.

Example 6 Impregnation of as-Pressed Composite with Phenolic Resin

An as-pressed carbon/carbon composite block prepared using a mixture of85% carbon fiber and 15% of a 155 SP coal tar pitch binder wasimpregnated with a diluted phenolic resin. The carbon/carbon compositehad reached a final temperature of ˜900° C. during the pressing. Thedensity of the untreated composite was measured as 1.52 g/cc.

The resin impregnant was prepared by dissolving a resole phenolic resinin a furfuraldehyde solvent at a 1:1 weight ratio. The dilution reducedthe viscosity of the resin from an original value of ˜500 cp to greaterthan about 100 cp at room temperature.

The impregnation was carried out by subjecting the composite to a vacuumof ˜0.4 mm followed by introduction of the resin/furfuraldehyde blendand equalization of the pressure to about 760 mm. The resin was cured byheating the impregnated composite to 250° C. in a vacuum oven. Aftercuring the composite had picked up 6.5 by weight and the compositedensity had increased to 1.62 g/cc.

The composite was then heat treated to 900° C. to convert the phenolicresin to carbon. The final density achieved was 1.58 g/cc representing a0.06 increase over the originally formed material.

Friction Testing of Impregnated C/C Composite.

As stated in Example 3, a carbon/carbon composite produced using 75% ofcarbon fibers and 25% of 170SP coal tar pitch was impregnated 2 timeswith pitch and then graphitized. The composite with a density of about1.70 g/cc was friction tested using the Chase Test. The compositeexhibited a low coefficient of friction of 0.1 at a low temperature ofabout less than 100° C. but the coefficient increased with increasedtemperature to a value of about 0.25 at about 400° C. The test resultsare represented on FIG. 3 as the composite without the frictionadditive. The composite did survive through the entire temperature rangewhen tested in friction against a cast iron part. Additionally thecomposite did not show the fade or loss of friction with increasingtemperature that is typical for conventional phenolic resin bondedfriction materials.

Example 7 Post Graphite Impregnation with Phenolic Resin

As-pressed composites were prepared with the composition of Example 6.One carbon/carbon composite block was heated to ˜3000° C. to effectgraphitization. A second composite block was pitch impregnated followedby rebake to 900° C. and graphitization. Both composites were thenimpregnated with the diluted phenolic resin used in Example 6 and heattreated to 250° C. to cure the resin.

The non-pitch impregnated composite with an initial density of 1.54 g/ccpicked up 7.7% of resin after curing while the composite that had beenpitch impregnated once, and had a baked density of 1.70 g/cc picked up5.2% of cured resin by weight. After impregnation and curing, thecomposite could be readily machined into thin strips with a thickness ofabout 250 microns. This was not possible for the non-treat componentimpregnated composite.

In the erosion test of a composite formed in accordance with example 7,the wear rate of the phenolic resin treated composite was measured asabout 3.5 wt. % when tested against a cast iron disc for a period ofabout 90 minutes. In contrast, a carbon/carbon composite produced by thesame process but without the phenolic resin treat had very high wearrate of about 13.9% when friction tested against cast iron in a 90minute time period.

The apparatus used to conduct the erosion test was a Friction AssessmentScreening Test (FAST) Machine. A description of the FAST machine and thetest may be located at the web cite for Link Testing Laboratories,http://www.linktestlab.com. The actual testing was performed by theCenter for Advanced Friction Studies at Southern Illinois University atCarbondale, Ill.

Example 8 Impregnation with Undiluted Phenolic Resin

In order to increase the phenolic resin content of the composite, animpregnation was carried out using the undiluted resole phenolic used inthe previous example. A carbon/carbon composite artifact 7.5×10 cm. wasprepared using a blend of 75% fibers and 25% of the 170 SP pitch.

After hot pressing the composite was heat treated at a graphitizationtemperature of ˜2600° C. The density of the graphitized composite wasmeasured as 1.31 g/cc. The composite was then impregnated with a liquidresole phenolic resin without the use of a diluent. The viscosity of thepure resin was ˜100 cp. In spite of the higher viscosity completeimpregnation of the composite with a total pickup of ˜35 weight % wasachieved. Composite specimens impregnated in this way were then cured attemperatures of 208 and 282 degrees C. The final density for thecomposites cured at 208° C. was measured as about 1.61 g/cc while thedensity for the 282° C. cured composites was about 1.58 g/cc. Thisdensity increase of ˜0.30 g/cc was substantially greater than achievedin previous examples.

Example 9 Impregnation with Furan Resin

A graphitized composite as prepared in example 8 was impregnated with ablend of furfuryl alcohol containing 10% of a 50% solution of zincchloride in water. The graphitized composite with a density of 1.42 g/ccpicked up about 16.2% by weight of the impregnant after curing at 280°C. to cure the resin, the density was increased to 1.65 g/cc afterimpregnation and curing.

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1-10. (canceled)
 11. A vehicle friction brake assembly comprising: a friction element having at least a metal surface which rotates with a wheel of a vehicle; and a braking element having a surface aligned to movably engage said metal surface of said friction element, wherein at least said surface of said braking element comprises a carbon composite having a carbonized matrix impregnated with a treating component.
 12. The vehicle friction brake assembly according to claim 11 wherein said surface further comprises at least one of a friction additive or a performance enhancer.
 13. The vehicle friction brake assembly according to claim 12 wherein a concentration of said friction additive through a thickness of said surface comprises substantially uniform.
 14. The vehicle friction brake assembly according to claim 11 wherein said treating component comprises at least one of a metal, a thermosett material, and combinations thereof.
 15. The vehicle friction brake assembly according to claim 11 wherein said friction element comprises a brake drum or a brake rotor.
 16. The vehicle friction brake assembly according to claim 11 wherein said braking element comprises a brake pad.
 17. The vehicle friction brake assembly according to claim 11 wherein said treating component comprises a thermosett material.
 18. A method of making a vehicle friction brake assembly comprising: rotatably attaching a friction element comprising a metal surface onto a vehicle; and aligning a braking element to movably engage said friction element, said braking element comprising a surface comprised of a carbon composite having a carbonized matrix and a treating component, said surface of said braking element aligned to engage said metal surface.
 19. The method according to claim 18, wherein said treating component comprises at least one of a thermosett material, a metal, a metal alloy, and combinations thereof.
 20. The method according to claim 18, wherein said composite further comprise at least one of a friction additive or a performance enhancer. 